A large amount of cerium oxide has conventionally been used as a co-catalyst for a catalyst for purifying exhaust gases, ceramics, and the like. In the field of catalysts, for example, properties of cerium oxide, which absorbs oxygen under the oxidizing atmosphere and desorbs oxygen under the reducing atmosphere, are utilized for improving the efficiency in purifying exhaust gases containing HC/CO/NOx as its components. In the field of ceramics, the cerium oxide is used in the form of a mixture or a compound with other elements as electrically conductive ceramics such as solid electrolyte, taking advantage of characteristic properties mentioned above.
However, though the conventional oxide mainly composed of cerium oxide has certain oxygen absorbing and desorbing capability, it cannot exhibit the capability sufficiently at about 600.degree. C. Further, at a temperature as high as 700.degree. C. or higher, the performance of the oxide is deteriorated.
In order to overcome such drawbacks, there is proposed a composite oxide mainly composed of cerium and zirconium. There are known, for example, a composite oxide containing cerium and zirconium (Japanese Laid-open Patent Application No. 4-334548), cerium dioxide containing zirconium having a large specific surface area prepared by adding 1to 20 weight % zirconium oxide to cerium oxide (Japanese Patent Publication No. 6-74145), and a cerium-zirconium composite oxide exhibiting the oxygen absorbing and desorbing capability of 100 .mu.mol/g or more at 400 to 700.degree. C. (Japanese Laid-open Patent Application No. 5-28672).
As a further improvement of the above-mentioned composite oxides, there is also proposed a composite oxide containing a third element in addition to cerium and zirconium. Examples of such composite oxide include, for example, a composite oxide composed of cerium, zirconium, and lanthanum (Japanese Laid-open Patent Application No. 6-154606), and a composite oxide containing cerium oxide, zirconium oxide, and hafnium oxide (Japanese Laid-open Patent Application No. 7-16452).
The conventional composite oxide containing cerium, however, is low in the degree of solid solution in the crystal phase. Further, a composite oxide is not known which can be reduced sufficiently under the reducing atmosphere at a temperature as low as 600 .degree. C. Accordingly, development of a composite oxide containing cerium oxide which exhibits sufficient oxygen absorbing and desorbing capability at low temperatures is demanded.
The conventional composite oxide containing cerium is generally prepared, for example, by a process including the steps of preparing a nitrate solution or a chloride solution containing cerium ions as well as zirconium ions, lanthanum ions, and hafnium ions which are necessary for composition; adding oxalic acid or an alkali compound such as ammonium bicarbonate to the solution to precipitate the metals mentioned above as a composite salt; and calcining the resulting precipitate. It is commonly known that the cerium ions used in the preparation of the composite oxide are trivalent cerium ions unless otherwise mentioned. The reason for this fact is explained, for example, in Inorganic Chemistry, New Edition, Volume 1, Toshizo Chitani, Sangyo Tosho Kabushiki Kaisha, p311 (1959). According to this reference, solutions of tetravalent cerium salts are prone to be oxidized very easily, and chlorides of tetravalent cerium easily release chlorine to become chlorides of trivalent cerium. Thus, solutions of cerium salts are stable when the cerium ions are trivalent, and therefore tetravalent cerium salts and solutions thereof are not usually marketed.
It is conventionally known that tetravalent cerium salts and solutions thereof can be obtained in the form of nitrates, sulfates, or composite salts of ammonium nitrates in the process of cerium purification, though such salts are unstable. However, it is not known widely to use the tetravalent cerium salts and the solutions thereof.